Carbon Dioxide and Methane Emissions from Mangrove-Associated Waters of the Andaman Islands, Bay of Bengal

Abstract

We estimated CO₂ and CH₄ emissions from mangrove-associated waters of the Andaman Islands by sampling hourly over 24 h in two tidal mangrove creeks (Wright Myo; Kalighat) and during transects in contiguous shallow inshore waters, immediately following the northeast monsoons (dry season) and during the peak of the southwest monsoons (wet season) of 2005 and 2006. Tidal height correlated positively with dissolved O₂ and negatively with pCO₂, CH₄, total alkalinity (TAlk) and dissolved inorganic carbon (DIC), and pCO₂ and CH₄ were always highly supersaturated (330–1,627 % CO₂; 339–26,930 % CH₄). These data are consistent with a tidal pumping response to hydrostatic pressure change. There were no seasonal trends in dissolved CH₄ but pCO₂ was around twice as high during the 2005 wet season than at other times, in both the tidal surveys and the inshore transects. Fourfold higher turbidity during the wet season is consistent with elevated net benthic and/or water column heterotrophy via enhanced organic matter inputs from adjacent mangrove forest and/or the flushing of CO₂-enriched soil waters, which may explain these CO₂ data. TAlk/DIC relationships in the tidally pumped waters were most consistent with a diagenetic origin of CO₂ primarily via sulphate reduction, with additional inputs via aerobic respiration. A decrease with salinity for pCO₂, CH₄, TAlk and DIC during the inshore transects reflected offshore transport of tidally pumped waters. Estimated mean tidal creek emissions were ∼23–173 mmol m⁻² day⁻¹ CO₂ and ∼0.11–0.47 mmol m⁻² day⁻¹ CH₄. The CO₂ emissions are typical of mangrove-associated waters globally, while the CH₄ emissions fall at the low end of the published range. Scaling to the creek open water area (2,700 km²) gave total annual creek water emissions ∼3.6–9.2 × 10¹⁰ mol CO₂ and 3.7–34 × 10⁷ mol CH₄. We estimated emissions from contiguous inshore waters at ∼1.5 × 10¹¹ mol CO₂ year⁻¹ and 2.6 × 10⁸ mol CH₄ year⁻¹, giving total emissions of ∼1.9 × 10¹¹ mol CO₂ year⁻¹ and ∼3.0 × 10⁸ mol CH₄ year⁻¹ from a total area of mangrove-influenced water of ∼3 × 10⁴ km². Evaluating such emissions in a range of mangrove environments is important to resolving the greenhouse gas balance of mangrove ecosystems globally. Future such studies should be integral to wider quantitative process studies of the mangrove carbon balance.